This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. There is significant pharmacological and behavioral evidence that group I metabotropic glutamate receptors (mGluR1a and mGluR5) in the nucleus accumbens play an important role in the neurochemical and pathophysiological mechanisms that underlie addiction to psychostimulants. During the reporting period, we used high resolution electron microscopic immunogold labeling method at the electron microscopic level to characterize changes in the subcellular and subsynaptic localization of both group I metabotropic glutamate receptors in the nucleus accumbens of cocaine-treated rats. After a single cocaine injection (30mg/kg) and 45 minutes withdrawal, there was a significant decrease in the proportion of plasma membrane-bound mGluR1a in accumbens shell dendrites. Similarly, the proportion of plasma membrane-bound mGluR1a was decreased in large dendrites of accumbens core neurons following chronic cocaine exposure (i.e. 1 week treatment followed by three weeks withdrawal). However, neither acute nor chronic cocaine treatments induced significant change in the localization of mGluR5 in accumbens core and shell, which is in contrast with the significant reduction of plasma membrane-bound mGluR1a and mGluR5 induced by local intra-accumbens administration of the group I mGluR agonist, DHPG. In conclusion, these findings demonstrated that cocaine-induced glutamate imbalance has modest effects on the trafficking of group I mGluRs in the nucleus accumbens. These results provided valuable information on the neuroadaptive mechanisms of accumbens group I mGluRs in response to cocaine administration.